A typical gas turbine engine combustor includes a generally annular chamber having a plurality of fuel injectors at an upstream head end. Combustion air is provided through the head and in addition through primary and intermediate mixing ports provided in the combustor walls downstream of the fuel injectors.
In order to improve the thrust and fuel consumption of gas turbine engines, i.e., the thermal efficiency, it is necessary to use high compressor pressure and combustion temperatures. This results in the combustion chamber experiencing high temperatures and there is a need to provide effective cooling of the combustion chamber walls. Various cooling methods have been proposed including the provision of a double walled combustion chamber whereby cooling air is directed into a gap between spaced outer and inner walls, thus cooling the inner wall. This air is then exhausted into the combustion chamber through apertures in the inner wall. The exhausted air forms a cooling film, which flows along the hot, internal side of the inner wall, thus preventing the inner wall from overheating.
The inner wall may comprise a number of heat resistant tiles. The tiles are generally rectangular in shape and are bowed to conform to the overall shape of the annular combustor wall. The tiles are conventionally longer in the circumferential direction of the combustor than in the axial direction.
It is known to provide pedestals, which extend from an outer surface of the tile towards the inner surface of the outer wall. The pedestals increase the surface area of the tile and facilitate heat removal from the tile “hot” side by primarily convection as cooling air passes between the pedestals and secondly by conduction from the pedestal to the outer “cold” wall of the combustor where the pedestal and wall contact.
The tiles and outer “cold” wall of the combustor are typically of cast construction. Cast components generally cannot be produced to very high tolerances and this inevitably results in gaps between some of the pedestals and the outer wall. Indeed, the pedestals, are typically arranged to provide a gap between the pedestal and the outer wall to prevent damage to the wall or tile caused by differences in thermal expansion between these two components.
Such a gap is undesirable since it reduces the effect of heat removal by conduction and additionally the effect of heat removal by convection since the air can pass over the pedestal tip rather than across the pedestal surface.